This invention relates to a combined summing and underspeed valve for use in hydraulic systems.
Many hydraulic systems used in work performing apparatus as, for example, hydraulic excavators, utilize a plurality of variable displacement pumps, one for each work function to be performed, driven by a single prime mover, such as an internal combustion engine.
In the usual case, the engine will be manually set to operate at a predetermined or normal operating speed and the various variable displacement pumps will be commanded to maximum displacement. In some cases, the loading on the engine may increase to the point where it begins to lug and its speed falls below the normal speed, while in other cases, one or more of the work performing systems may be overloaded, causing the development of a high back pressure within its individual hydraulic system.
In either case, it is desirable to relieve the overload condition by lessening the displacement of the pumps.
Conventionally, systems employed for accomplishing the foregoing function have utilized two valves, one being an underspeed valve and the other being a summing valve. See, for example, U.S. Letters Pat. No. 3,841,795, issued Oct. 15, 1974 to Ferre et al. This approach, while perfectly satisfactory in terms of accomplishing the intended function, is expensive in that two valves, a summing valve and an underspeed valve, must be utilized.